I have no experience with "Marktplaat". Is it fairly easy to set up a payment method? Is international shipping common practice?
Marktplaats is the biggest 2nd hand website in the Netherlands. Most of the things on there are in an auction format, even if you see a listed price. That is kinda the price the seller aims at, and they can also set a minimum bid price. If you place a bid, it is up to the seller to agree, or wait for other offers. There's no expiry date on the listings.
International shipping is fairly common, but always good to ask when you place your bid. (When you bid it lets you send a custom message to the seller)
Most people will speak English, so I'd say you're safe messaging in English if you don't speak Dutch.
Sadly, I found out that Markplaats requires a phone # to complete account registration, and it does not allow for US telephone numbers. I tried circumventing that with a temporary SMS phone # websites, but I couldn't get any of them to actually work (they never received the text msg from Markplaats). That's a real bummer.